In order to maximize the information content transmitted over a prescribed spectral bandwidth, polarization multiplexing (also known as “dual-polarization”) is being increasingly used with new transmission formats. The underlying idea is that the spectral density (conveniently measured in units of bits/Hz) can be effectively doubled by employing two orthogonally polarized data-carrying signal components sharing the same optical signal bandwidth. Normally, these two orthogonally polarized components are transmitted with approximately the same intensity, rendering the total resultant light effectively unpolarized as seen from a test and measurement instrument having an electronic detection bandwidth significantly lower than the symbol rate of the polarization-multiplexed signal, such as is normally the case with Optical Spectrum Analyzers (OSA).
The Optical Signal-to-Noise Ratio (OSNR) is a conventional measurable characteristic of the quality of a signal carried by an optical telecommunication link. Under normal and proper operating conditions, the OSNR of an optical communication link is typically high, often in excess of 15 dB or 20 dB, or even greater. The dominant component of the noise in an optical communication link is typically unpolarized Amplified Spontaneous Emission (ASE) noise, which is a spectrally broadband noise source contributed by the optical amplifiers in the link.
A method of measuring the noise level on polarization-multiplexed signals using an acquired optical spectrum trace of the signal was proposed by Gariepy et al. (see US Patent Application Publication US 2012/0201533 A1, commonly owned by the Applicant and hereby incorporated by reference). This method is based on the knowledge of the spectral shape of the data-carrying signal contribution provided by a reference signal. Based on this knowledge, the data-carrying signal contribution and the ASE-noise contribution that otherwise appear as merged together on the optical spectrum trace, may be mathematically discriminated from one another. Knowledge of the spectral shape of the signal contribution may derive from acquisition of a reference signal taken, for example, at a different point, generally upstream, along the same optical communication link (i.e. the reference signal originates from the same optical transmitter), where the OSNR is known or where the signal can be considered free of ASE noise. This method assumes that, within the optical signal bandwidth, the spectral shape of the signal does not significantly change along the communication link. The signal contribution of such a reference signal is therefore spectrally representative of the signal contribution of the signal-under-test.
However, spectral deformations mostly induced by Non-Linear Effects (NLE) have become more frequent in the case of new deployments using polarization multiplexing, because optimum performance in Bit Error Rate (BER) is obtained by increasing the power propagating within the optical fiber to such an extent that non-linear effects can no longer be neglected. The method proposed by Gariepy et al (loc cit) is impacted by signals subjected to NLE-induced spectral deformations. Consequently, overall system performance in terms of BER is not only affected by the ASE-noise level but also from such non-linear effects (see Vacondio et al, “On nonlinear distorsions of highly dispersive optical coherent systems”, Optics Express, Vol. 20, No. 2, pp. 1022-1032 (2012)). Signal quality therefore cannot be assessed based only on the conventionally measured ASE noise level because proper performance indicators should also account for NLE-induced distortions.
A method of measuring the noise level on polarization-multiplexed signals in presence of NLE-induced signal deformation was proposed by Gariepy et al. (see US Patent Application Publication US 2014/0328586 A1, commonly owned by the Applicant and hereby incorporated by reference). The method is based on an analysis of the power spectral density of the Signal-Under-Test (SUT) and is predicated upon knowledge of the spectral shape of the signal in the absence of significant noise or spectral deformation. Again, this knowledge is provided by a reference optical spectrum trace. Based on this knowledge and under the assumption that ASE noise level is approximately constant in wavelength over a given spectral range (i.e. the ASE noise variation is negligible compared to the signal contribution variation), the spectral deformation of the signal contribution of the SUT may be estimated using a comparison of the spectral variations of the optical spectrum trace of the SUT with that of the reference optical spectrum trace.
However, it has been found that this method has limited performance in some cases of tight filtering caused by optical add/drop multiplexers combined with wide-spectrum optical communication signals such as RZ formats and pulse-shaped signals (i.e. when the optical signal bandwidth of the optical communication signal is wide compared to the bandwidth of the ROADMs). In such cases, add-drop filtering leads to non-negligible spectral deformation of the optical communication signal along the communication link (typically in the “wings” of the signal), which makes it difficult to discriminate ASE noise from NLE-induced spectral deformations.
There is therefore a need for a method to characterize polarization-multiplexed signals subject to NLE-induced and/or add-drop filtering-induced signal deformation.